1. Field of the Invention
The present invention relates to an electrostatic latent image developer to be used in an electrophotographic method and electrostatic recording, and to an electrostatic latent image developing carrier to be used in the developer.
2. Description of the Related Art
In an electrophotographic method, an electrostatic latent image is formed on a latent image holding member (photoreceptor) by charging and exposure steps, and developed by toner. The developed image is transferred to an image receiving member and fixed by heating or the like to obtain an image. Developers usable in such an electrophotographic method may be roughly classified into one-component developers and two-component developers. A one-component developer is a toner itself containing a colorant dispersed in a binder resin, and a two-component developer comprises such a toner and a carrier. The two-component developer has high controllability because the carrier has the functions of stirring, transferring and charging the developer; that is the functions as a developer is separated. For the above reasons, the two-component developer is widely used at present.
In recent years, digitalization has been adopted as a measure to achieve high image quality. This digitalization enabled high-speed processing of a complicated image. Moreover, a laser beam is used to form an electrostatic latent image on a latent image holding member. The development of exposure technologies using a small laser beam has increased the resolution of an electrostatic latent image. Image processing technologies like this permit the application of electrophotographic method to light printing. Moreover, there has been a demand for developments of high-speed and high-resolution electrophotographic devices in recent years. In particular, regarding full-color image quality, high-quality image close to that of high-class printing or silver salt photography is desired. Therefore, the retention of the charge of the developer is important to ensure the visualization of latent images with higher resolution for a long period of time. Namely, there are needs for further improvement of the charge retention property of the carrier having a charging function.
The resistance of the carrier is also an important factor to achieve high quality image. In order to improve the image quality, the carriers used in current digital devices have been made smaller and their electric resistance values have also been reduced. These small-sized carriers make it possible to reproduce a precise image and to provide stable charges even to small-sized toners. Also, the reduction in resistance improves reproduction of a solid image. Such carriers are particularly preferable for the formation of full-color high-density images.
If a carrier having high resistance is used, the quality a half-tone image is deteriorated. For example, image defects may occur in which white portion is generated at the boundary between a black-haired person image and a pale background. This is the reason why a carrier having a low resistance has been selected in high image quality full color machines so as to fully utilize the image characteristics of the machines (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-39547, 10-133480, and 2003-280284). However, on the other hand, there are many techniques in which the resistance of carriers is increased so as to limit the transfer of a carrier (see, for example, JP-A No. 7-271106).
Although the use of a carrier having a low resistance remarkably improves image qualities, the transfer of the carrier to an image easily occurs.
The carrier transfer is roughly divided into three categories: (i) carrier transfer onto the entire surface of the output image, (ii) carrier transfer onto the background, and (iii) carrier transfer onto the image. The main cause of the carrier transfer onto the entire surface is low magnetic force. The carrier transfer onto the background is usually caused by high resistance and a large particle diameter; specifically, a carrier is oppositely charged by a toner, and develops uncharged areas. The carrier transfer onto the image is mainly caused by low resistance; the toner charge or developing charge is injected into the carrier, and the carrier, together with the toner, develops the charged area.
Therefore, when the resistance of the carrier is decreased, image quality is improved but carrier transfer to the image is caused by the injection of charge. Therefore, the carrier design has aimed to achieve a resistance within such a range to cause no carrier transfer and to enable formation of high-quality images.
Carriers are roughly classified into (i) dispersion-type carriers obtained by dispersing magnetite in resins and (ii) resin-coated carriers each obtained by coating the surface of a core such as ferrite, magnetite or iron powder with a resin. The developer containing a carrier of the former class has reduced fluidity and has inferior conveyability to developers containing carriers of the latter class, partly owing to the reduction in specific gravity. Also, because the a carrier of the former class has a lower magnetic force per carrier particle than a developer of the latter class, the carrier of the former class is disadvantageous in carrier transfer. Therefore, various studies have been made on resin-coated carriers. For example, a carrier has been proposed which has a low-resistance layer provided on the surface of a core and a high-resistance layer thereon (see, for example, JP-A No. 2004-61730). Although this carrier is surely more resistant to the abrasion of the coated layer (resin coating layer) over time, but has no effect on the prevention of peeling of a part with low adhesion to the core. As a result, the core is exposed to be injected with charge, thereby generating carrier transfer. Also, an attempt is made to improve the adhesion to the core by defining the diameter of pores on the surface of the core (see, for example, JP-A No. 2-135371). However, when the shape of the core is complex as in the above method, the core easily cracks or chips from the deformed part. Even if the core is coated with a resin, the resulting carrier easily cracks, or the core cracks during the process of producing the carrier. Also, another carrier has been proposed which has a porous core (see, for example, JP-A No. 2004-77568). However, the core itself has insufficient strength and the coating layer does not penetrate to the inside. Therefore, the carrier is not resistant to cracking, whereby the carrier is transferred by the injection of charge.